Superionic-like diffusion in an elemental crystal: bcc Titanium

TL;DR

This study uncovers superionic-like diffusion in bcc titanium at ambient conditions, revealing a collective atom migration mechanism similar to that in superionic conductors and liquid metals, supported by first-principles simulations.

Contribution

It demonstrates for the first time that a superionic-like diffusion mechanism occurs in pure bcc titanium at high temperatures and ambient pressure, linking atomic dynamics to phonon behavior.

Findings

Collective atom migration along closed-loop paths in bcc Ti.

Diffusion mechanism resembles superionic conductors and liquid metals.

Explains non-Arrhenius diffusivity and low-frequency phonons in bcc Ti.

Abstract

Recent theoretical investigations [Belonoshko et al. Nature Geoscience 10, 312 (2017)] revealed occurrence of concerted migration of several atoms in bcc Fe at inner-core temperatures and pressures. Here, we combine first-principles and semi-empirical atomistic simulations to show that a diffusion mechanism analogous to the one predicted for bcc iron at extreme conditions is also operative and of relevance for the high-temperature bcc phase of pure Ti at ambient pressure. The mechanism entails a rapid collective movement of numerous (from two to dozens) neighbors along tangled closed-loop paths in defect-free crystal regions. We argue that this phenomenon closely resembles the diffusion behavior of superionics and liquid metals. Furthermore, we suggest that concerted migration is the atomistic manifestation of vanishingly small w-mode phonon frequencies previously detected via neutron scattering and the mechanism underlying anomalously large and markedly non-Arrhenius self-diffusivities characteristic of bcc Ti.